Microbial diseases of plants are a significant problem to the agricultural and horticultural industries. Plant diseases in general annually cause millions of tonnes of crop losses with fungal and bacterial diseases responsible for significant portions of these losses. One possible way of combating fungal and bacterial diseases is to provide transgenic plants capable of expressing a protein or proteins that in some way increase the resistance of the plant to pathogen attack. A simple strategy is to first identify a protein with anti-microbial activity in vitro, to clone the DNA sequence encoding the protein, to make a chimeric gene construct for efficient expression of the protein in plants, to transfer this gene to transgenic plants and to assess the effect of the introduced gene on resistance to microbial pathogens by comparison with control plants.
The first and most important step in the strategy for disease control described above is to identify a protein with strong anti-microbial activity. In recent years, many different plant proteins with anti-microbial and/or antifungal activity have been identified and described. These proteins have been categorized into several classes according to either their presumed mode of action and/or their amino acid sequence homologies. These classes include the following: chitinases (Roberts, W. K. et al. [1986] Biochim. Biophys. Acta 880:161-170); β-1,3-glucanases (Manners, J. D. et al. [1973] Phytochemistry 12:547-553); thionins (Bolmann, H. et al. [1988] EMBO J. 7:1559-1565 and Fernadez de Caleya, R. et al. [1972] Appl. Microbiol. 23:998-1000); permatins (Roberts, W. K. et al. [1990] J. Gen. Microbiol. 136:1771-1778 and Vigers, A. J. et al. [1991] Mol. Plant-Microbe Interact. 4:315-323); ribosome-inactivating proteins (Roberts, W. K. et al. [1986] Biochim. Biophys. Acta 880:161-170 and Leah, R. et al. [1991] J. Biol. Chem. 266:1564-1573); plant defensins (Terras, F. R. G. et al. [1995] The Plant Cell 7:573-588); chitin binding proteins (De Bolle, M. F. C. et al. [1992] Plant Mol. Biol. 22:1187-1190 and Van Parijs, J. et al. [1991] Planta 183:258-264); thaumatin-like, or osmotin-like proteins (Woloshuk, C. P. et al. [1991] The Plant Cell 3:619-628 and Hejgaard, J. [1991] FEBS Letts. 291:127-131); PR1-type proteins (Niderman, T. et al. [1995] Plant Physiol. 108:17-27); the non-specific lipid transfer proteins (Terras, F. R. G. et al. [1992] Plant Physiol. 100:1055-1058 and Molina, A. et al. [1993] FEBS Letts. 3166:119-122); and, the knottin or knottin-like proteins (Cammue, B. P. A. et al. [1992] J. Biol. Chem. 67:2228-2233). In addition, plants are not the sole source of anti-microbial proteins and there are many reports of the isolation of anti-microbial proteins from animal and microbial cells (reviewed in Gabay, J. E. [1994] Science 264:373-374 and in “Anti-microbial peptides” [1994] CIBA Foundation Symposium 186, John Wiley and Sons Publ., Chichester, UK).
There is some evidence that the ectopic expression of genes encoding proteins that have in vitro anti-microbial activity in transgenic plants can result in increased resistance to microbial pathogens. Examples of this engineered resistance include transgenic plants expressing genes encoding: a plant chitinase, either alone (Broglie, K. et al. [1991] Science 254:1194-1197) or in combination with a β-1,3-glucanase (Van den Elzen, P. J. M. et al. [1993] Phil. Trans. Roy. Soc. 342:271-278); a plant defensin (Terras, F. R. G. et al. [1995] The Plant Cell 7:573-588); an osmotin-like protein (Liu, D. et al. [1994] Proc. Natl. Acad. Sci. USA 91:1888-1892); a PR1-class protein (Alexander, D. et al. [1993] Proc. Natl. Acad. Sci. USA 90:7327-7331) and a ribosome-inactivating protein (Logemann, J. et al. [1992] Bio/Technology 10:305-308).
Although the potential use of anti-microbial proteins for engineering disease resistance in transgenic plants has been described extensively, there are other applications which are worthy of mention. Firstly, highly potent anti-microbial proteins can be used for the control of plant disease by direct application (De Bolle, M. F. C. et al. [1993] in Mechanisms of Plant Defence Responses, B. Fritig and M. Legrand eds., Kluwer Acad. Publ., Dordrecht, NL, pp. 433-436). In addition, anti-microbial peptides have potential therapeutic applications in human and veterinary medicine. Although this has not been described for peptides of plant origin it is being actively explored with peptides from animals and has reached clinical trials (Jacob, L. and Zasloff, M. [1994] in “Anti-microbial Peptides”, CIBA Foundation Symposium 186, John Wiley and Sons Publ., Chichester, UK, pp. 197-223).
The invention described herein constitutes a previously undiscovered and novel protein with anti-microbial activity. This protein can be isolated from Macadamia integrifolia (Mi) plants. Macadamia integrifolia belongs to the family Proteaceae. M. integrifolia, also known as Bauple Nut or Queensland Nut, is considered by some to be the world's best edible nut. For this reason it is extensively cultivated commercially, both in Australia and overseas (Williams, Keith A. W., Native Plants (Queensland), Volume II, 1984, published by Keith A. W. Williams and printed by Printcraft of Newstead, Qld, Australia).
An object of the invention is to provide a protein having anti-microbial activity, homologues of the protein, and variants of the protein.
A further object of the invention is to provide DNA encoding anti-microbial protein, homologues of the proteins and variants of the protein.
Yet another object of the invention is to provide DNA constructs which include DNA encoding anti-microbial protein, homologues of the protein and variants of the protein.
Still further objects of the invention are to provide cells and plants harbouring a DNA construct which includes DNA encoding anti-microbial protein, homologues of the protein and variants of the protein, and to provide reproductive tissue of said plant.
Additional objects of the invention are to provide compositions comprising the anti-microbial protein, homologues and variants, and methods of treating plants and humans for microbial infestation using the proteins and compositions according to the invention.